A flow control valve serving alternatively as a flow-dividing valve and as a flow-combining valve is already known from U.S. Pat. No. 3,554,213.
This flow control valve comprises a valve body which has a longitudinally extending bore for receiving therein a longitudinally extending main spool, for sliding motion. A coil spring urges the main spool to be positioned in the centre of the bore in said valve body.
Two subsidiary spools are disposed in end-to-end relation in an axially extending passageway provided in said main spool for sliding motion relative to each other.
The valve body has a first port connected to a source of hydraulic fluid (serving alternatively as an inlet and an outlet port) and a pair of second ports connected to a hydraulic device and through which hydraulic fluid can flow in one direction or in the reverse one.
Each of the subsidiary spools has a partition wall with an orifice formed therein and a passage therethrough adapted to selectively register with one passage of each of two pairs of passages formed in the main spool and one formed in the valve body.
When the flow control valve operates in dividing mode, the flow of hydraulic fluid introduced in the first port flows inside the main spool where it is divided into two streams into each subsidiary spool. As the fluid passes through the orifice formed in the partition wall of said subsidiary spool, the resistance offered by the orifices to the fluid flow causes the subsidiary spools to move away from each other. The hydraulic fluid flows through the passage provided in the subsidiary spool and then in through the passages provided in the main spool to be discharged through the second ports of the valve body.
In case the pressures in the two second ports are not equal then the main spool will move to the right or to the left till the pressures inside the two subsidiary spools are equalized. Then the main spool will come back to its central position.
When the flow control valve operates in combining mode, the flow of hydraulic fluid introduced in the two second ports are combined inside the passageway of the main spool before to be discharged through the first port of the valve body. In this case, the two subsidiary spools move towards each other.
Again, in case the pressures in the two second ports are not equal then the main spool will move to the right or to the left. The movement of the main spool (for example the rightward movement), gradually reduces the degree of opening of the right passage of the left pair of passages provided in the main spool and increases the difference of pressure between the left second port and the fluid chamber of the left subsidiary spool. This results in a reduction of pressure of fluid inside the left subsidiary spool. When the pressure of hydraulic fluid introduced into the fluid chamber of the left subsidiary spool through the left port is lower than the pressure of hydraulic fluid introduced into the fluid chamber of the right subsidiary spool through the right port, then the main spool will move to the left.
The rightward and leftward movements of the main spool are alternatively repeated rapidly to maintain the main spool in the normal central position.
However, such kind of flow control valve, in combining mode and for high fluid flow rate has the disadvantage to be less accurate because of the oscillations of the main spool.
Further, theoretically the pressure drop through the orifices provided in each partition wall of said subsidiary spools should be independent from main spool axial position. Nevertheless, because of high turbulent flow, this is not always true.